In a mixer for a microwave circuit, a microstrip line hereinafter "strip line," is formed on a printed circuit board. The strip line is used as a transducer without a high-frequency core to promote miniaturization of a device and to have a stable characteristic, etc.
FIG. 1(A) is a partial diagram of a conventional mixer circuit. The strip lines 61 and 62 are formed on the surface of a printed circuit board. However, strip lines 63 and 64 represented by oblique lines are referred to as "ground lines" and are formed on the back surface of the printed circuit board. These ground lines serve as a transducer in combination with the strip lines 61 and 62. The ground line 63 and 64 are both continuous with a ground plane GND. However, the ground line 64 is continuous with the ground plane GND only at one end where a diode ring 70 is located.
As shown in FIG. 2, the diode ring 70 comprises four diodes which are connected to one another in a ring form. Respective connection points of the diode ring 70 thereof serve as four terminals a', b', c' and d' (in this case, such that two diodes are crossed).
Now referring back to FIG. 1, the tip of the strip line 61 is connected to one end a' of the diode ring 70, and one end of the strip line 62 is connected to another terminal b' of the diode ring 70. The other end of the strip line 61 which serves as an RF input terminal is connected through a hole 85 to the tip of the ground line 64. The other end of the strip line 62 is connected through a hole 86 to the ground plane GND.
Other terminals c' and d' of the diode ring 70 of FIG. 1(A) are connected to both ends 91a and 91b of one winding of a transducer 90 as shown in FIG. 1(B). An IF terminal is drawn out of the center thereof. Both ends 92a and 92b of the other winding of the transducer 90 whose center is grounded are supplied with a local oscillator LO through a common mode choke 93.
The mixer as shown in FIG. 1(A) may be a mixer of Model No. ZFM-4212 from American Mini Circuit Co., Ltd. and can be used in a microwave band of about 4 GHz maximum frequency. Such a mixer is a double-balanced type. Although the primary conversion loss of the mixer is about 6 dB, the secondary distortion is increased due to a possible coupling between the diodes. Thus, the mixer is not usable for an application requiring low distortion. In other words, although a highly-symmetrical pattern can be formed on a printed circuit board, the diodes must be arranged in such a way as described above to result in a high distortion. On the other hand, if a diode ring is designed to substantially eliminate coupling between the diodes as shown in FIG. 2. The crossing of the wiring pattern must be required, and the symmetry of the circuit itself is lost. However, the symmetry of the diode ring itself can be maintained.
Therefore, a large number of microwave mixers adopt a single-balanced type as shown in FIG. 3. In FIG. 3, a series connection of diodes 71 and 72 are connected between the terminals 91a and 91b of one winding of the transducer 90 such as shown in FIG. 1(B). The connection point of the diodes 71 and 72 serves as an RF input terminal. However, such a single-balanced mixer has a high conversion loss of 9 dB. In addition, it is more deteriorated in Noise Figure (NF) than the double-balanced typed.